Support PreToolUse additionalContext (#20692)

# Why

`PreToolUse` already exposes `hookSpecificOutput.additionalContext` in
the generated hook schema, but the runtime still rejected it as
unsupported. That leaves `PreToolUse` out of step with the other
context-injecting hooks and prevents hook authors from attaching
model-visible guidance to a pending tool call before it runs.

# What

- Parse `PreToolUse.additionalContext` and carry it through the hook
event pipeline.
- Record `PreToolUse` context at the hook boundary so successful context
is preserved for both allowed and blocked calls without widening the
tool registry surface.
- Preserve existing deny behavior when context is combined with either
`permissionDecision: "deny"` or the legacy `decision: "block"` shape.

codex-rs/core/src/hook_runtime.rs

@@ -161,8 +161,10 @@ pub(crate) async fn run_pre_tool_use_hooks(
         hook_events,
         should_block,
         block_reason,
+        additional_contexts,
     } = hooks.run_pre_tool_use(request).await;
     emit_hook_completed_events(sess, turn_context, hook_events).await;
+    record_additional_contexts(sess, turn_context, additional_contexts).await;
 
     if should_block {
         block_reason.map(|reason| {